Generally, an object of a car air conditioning system is to maintain a comfortable indoor condition to improve ride comfort of a driver and a passenger. In the car air conditioning system, an electronic control unit (ECU) operates an air conditioner (A/C) compressor in connection with a blower by a manipulation of an A/C air conditioning switch of a user, thereby using the A/C compressor in hot summer or in the case in which indoor humidity needs to be controlled. The A/C compressor is operated by the manipulation of the user in principle, but is also operated to control an indoor temperature in a case corresponding to a temperature condition set by the user in a vehicle in which a full auto temperature control (FATC) system is mounted.
Currently, in the operation of the A/C compressor, when only a refrigerant pressure condition is satisfied regardless of a driving condition of an engine in the case in which the driver or the passenger operates an A/C switch or operates a front glass moisture removing mode, the ECU engages the A/C compressor with the engine to compress an A/C refrigerant through the A/C compressor. In this case, a predetermined amount of fuel needs to be injected in order for the engine to maintain a predetermined revolutions per minute (RPM), such that fuel consumption may be deteriorated.
That is, when the A/C compressor is operated in an idle rotation region after warming up the engine in a vehicle, fuel is further consumed by 60% as compared with the case in which the A/C compressor is not engaged with the engine. In this case, it is assumed that the engine is operated in the idle rotation region, and a ratio of an amount of fuel consumed by the A/C compressor to a total amount of sprayed fuel will be relatively reduced in the case in which the engine is operated under a general driving condition. However, when the vehicle is driven in summer in a city in which traffic congestion is severe, a fuel consumption difference depending on turn-on/off of the AC will be large.